Recently, a new class of wireless sensing systems have been developed that use open-circuit, electrically-conductive spiral trace sensors. Details of these sensors and sensing systems are described in U.S. Pat. No. 8,430,327. Briefly, the described wireless sensing system includes a sensor made from an electrical conductor shaped to form an open-circuit, electrically-conductive spiral trace having inductance and capacitance. In the presence of a time-varying magnetic field, the sensor resonates to generate a harmonic response having a frequency, amplitude and bandwidth. A magnetic field response recorder wirelessly transmits the time-varying magnetic field to the sensor and wirelessly detects the sensor's response.
The above-described wireless sensing technology provides a new technical framework for designing, powering, and interrogating sensors. These unique sensors can detect physical changes in the environment or any material placed within the near field (i.e., millimeters to tens of centimeters) of the sensor. Detected changes are generally associated with a localized change in a material's permittivity, permeability, and/or conductivity. The material may be any state of matter, plasma, gas, liquid, or solid. Changes to a material's state cause disturbances in the wireless sensor's magnetic field that can be sensed by a magnetic field response recorder. Since the sensor's magnetic field is limited to the near field, the recorder's antenna must also be in the sensor's near field, thereby limiting the number of applications that can use this technology.